In data recording and transmission a multitude of improvements in all areas of the technology have contributed to drastic reductions in the cost of recording or transmitting an individual bit. Among the areas providing this contribution is that related to the encoding and decoding of the data involved. There have been a number of coding techniques developed that have progressively reduced the number of flux reversals or signal changes necessary for each information bit involved. The problem has always been complicated by the fact that the data itself must contain enough clock information so that the readback or receiving circuitry can synchronize with the data being read back or received in the face of noise and gradual frequency variations in the data. In these codes, individual flux reversals or signal changes create a continuous output signal, with the information content present in the time of individual reversals.
One of the most efficient ways of encoding such data is known as ternary encoding, which is characterized by two classes of signal elements, singlet pulses and doublet pulses. It is convenient to consider the frequency of the data clock for ternary encoding as being established by a single cell time. A singlet pulse is generated by a single, relatively isolated, flux or signal transition occupying at least 3 cell times, the polarity of each singlet pulse on readback being opposite that of the previous singlet. The polarity of a singlet pulse is established by the direction in the medium or channel of the flux reversal or signal change, which generates the singlet pulse during readback. A doublet pulse comprises 2 flux reversals of opposite polarity spaced 1 cell time apart, whose mutual interference create a pulse of at least 4 cell times and create 2 closely spaced peaks of opposite polarity with a zero crossing transition between them. For purposes of synchronizing these pulses in the clock in the readback circuitry, a singlet pulse is considered to occur in the middle of the 3 pulses which it occupies, while a doublet is considered to occur at the zero crossing between the 2 opposite polarity peaks which form it. Because each doublet is formed of 2 flux reversals of opposite direction, it is clear that the flux reversals for successive singlets are of opposite polarity regardless of the number of doublets interposed between them.
Although the original flux reversals during writing which create the pulses on readback are nominally of the same strength, the greater spacing between each singlet and adjacent pulses than between half-cycles of a doublet causes each singlet on readback to have substantially greater duration and magnitude than a doublet half-cycle. This makes reliable detection of a singlet much easier than detection of a doublet.
Noise and pulse crowding can cause doublets to be improperly detected. A known way of screening out some of the false zero crossings created by noise is, during signal processing, to compare the slope of the transition portion of a doublet between the peaks, with the polarity of the most recent singlet. One can see that the polarity of this slope and the most recent singlet should be the same. It is possible, however, that noise in the readback signal can occasionally cause data to be read back falsely with compatible slope polarity, and hence produce errors.
It should be understood that many types of ternary codes can be devised, all having a different relationship with respect to the sequence of singlet and doublet pulses and the spacing between them. In fact, a typical ternary code may permit as many as 8 cell times between the actual time of arrival of 2 adjacent pulses, singlet or doublet. It is particularly during these gaps that noise can intrude to cause the readback errors mentioned.
It should also be understood that other types of data encoding may employ doublets. For example, vertical recording techniques use only doublet pulses to carry information, making reliable detection of these pulses even more critical.
As to prior art, U.S. Pat. No. 3,631,263 (Graham et al.) discloses apparatus which is related to the invention to be disclosed, but which solves a somewhat different problem. In the Graham et al. patent, peak detection in combination with peak-to-peak voltage transition discrimination is used to reject noise having relatively small peak-to-peak transitions.